The terpenoids, also called isoprenoids, constitute the largest family of natural products with over 22,000 individual compounds of this class having been described. The triterpenes or terpenoids (hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, polyprenols, and the like) play diverse functional roles in plants as hormones, photosynthetic pigments, electron carriers, mediators of polysaccharide assembly, and structural components of membranes. The majority of plant terpenoids are found in resins, latex, waxes, and oils.
Two molecules of farnesyl pyrophosphate are joined head-to-head to form squalene, a triterpene, in the first dedicated step towards sterol biosynthesis. Squalene is then converted to 2,3-oxidosqualene which, in photosynthetic organisms, may be converted to the 30 carbon, 4 ring structure, cycloartenol or to the 5 ring homolog, β-amyrin, a sapinogenin precursor. This conversion step is catalyzed by one of at least two oxidosqualene cyclases: cycloartenol synthase or β-amyrin synthase.
Cycloartenol is formed by the enzyme cycloartenol synthase (EC 5.4.99.8), also called 2,3-epoxysqualene-cycloartenol cyclase. The basic nucleus of cycloartenol can be further modified by reactions such as desaturation or demethylation to form the common sterol backbones such as stigmasterol and sitosterol, which can be modified further.
The β-amyrin cyclization activity is distinct from cycloartenol synthase (Kushiro, T., et al. (1998) Eur. J. Biochem. 256:238–244). β-amyrin synthase catalyzes the cyclization of 2,3-oxidosqualene to β-amyrin. Yet, the basic β-amyrin ring structure may be modified in much the same manner as is the cycloartenol structure to give classes of sapogenins. Saponins are glycosylated sapogenins and may play a pathogen defense role in plant tissues.
Soybean seeds, for example, contain several classes of saponins, all of which are formed from one sapogenin ring structure that is modified by hydroxylation and by different carbohydrate moieties. Total saponin content varies somewhat by soybean cultivar but is in the range of 0.25% of the seed dry weight (Shiraiwa, M., et al. (1991) Agric. Biol. Chem. 55:323–331).
The name saponin was derived from their strong foaming power. The physiological function of saponins in soybean seeds is not clear, but they do contribute to the bitter or astringent flavor of soybean seeds (Okubo, K., et al. (1992) Biosci. Biotechnol. Biochem. 56:99–103). Saponins are thought to have cholesterol-lowering effects and reduction in colon cancer risk. Besides imparting undesirable flavors to feed and foods, saponins have been shown to have hemolytic action against red blood cells. Soybeans are also involved in the reduction of hot flashes in postmenopausal women, lowering the risk of hormone-related cancer, slowdown of bone loss in osteoporosis and improvement in vascular health. Saponins are believed to be involved in these beneficial soybean effects.
A variety of processed vegetable protein products are produced from soybean. These range from minimally processed, soy beans and soy nuts such as toasted soy nuts and defatted items such as soybean meal, grits, and flours to more highly processed items such as soy protein concentrates and soy protein isolates. In other soy protein products, such as full-fat soy flour, the oil is not extracted. In addition to these processed products, there are also a number of specialty products based on traditional Oriental processes, which utilize the entire bean as the starting material. Examples include soy milk, soy sauce, tofu, natto, miso, tempeh, and yuba.
Examples of use of soy protein products in human foods include applying soy protein concentrates and soy protein isolates in nutritional beverage, emulsified meat and whole muscle meat applications; textured soy protein in meat analogues; soy protein isolates in infant formula. Facilities and methods to produce protein concentrates and isolates from soybeans are available across the world. To the extent that they are retained in these processed soy fractions and the foods prepared from them, the saponin content of the starting beans influences the flavor of the food.
Sequences of two different β-amyrin synthase isoforms has been described for Korean ginseng (Panax ginseng; NCBI General Identifier Nos. 3721856 and 3688600). A soybean EST having NCBI General Identification No. 5606831 has been identified with and is “similar to the ginseng sequence,” according to the NCBI entrez.
Identification of the genes encoding oxidosqualene cyclases in a variety of crops will allow the manipulation of the same. Interference with triterpenoid ring synthesis during plant development may be expected to decrease the total content of saponins in plant parts resulting in foods with increased nutritional value, and better flavor.